The principal objective of the proposed program is to determine how prothrombin activation is regulated in vitro. The investigation will be specifically directed to discovering how Factor Va accelerates thrombin formation and by its own activation and inactivation may control the prothrombin activation process. The question of whether plasma protease inhibitors such as Antithrombin III, alpha-1-antitrypsin and alpha-2-macroglobulin function in controlling prothrombin activation or serve mainly as "sinks" for permanently inactivating Factor Xa and thrombin such as might occur prior to their removal from the circulation will also be investigated. Studies on the relative specificity of these inhibitors for Factor Xa and thrombin and the effect of heparin on the inhibition reactions will form a background repertoire of information for investigating the role of the inhibitors in the activation process. Factor V will be chemically characterized and the mechanism of its activation will be determined. Detailed investigation of the kinetics of prothrombin activation will be made from both initial rate studies and complete activation time course monitoring. Computer simulation will be used in both designing experiments and interpreting the activation experiment results.